A braking device for a motorcycle is known wherein a wheel braking means at a rear wheel is interlocked with a brake operation at a front wheel. For example, when a brake lever at the front wheel is operated, braking pressure acts upon a brake caliper at the front wheel via a master cylinder, a part of pressure that acts upon the master cylinder also acts upon a brake caliper at the rear wheel via a proportioning valve, and the side of the rear wheel is braked. See, JP-A No. 368267/1992.
A brake system in which front and rear wheel braking means brake together by applying a brake at one of front and rear wheels will be called a combined brake system (CBS) below.
However, the conventional type CBS in which pressure is supplied to the brake caliper at the rear wheel via the proportioning valve has a problem in that the braking pressure of the front and rear wheels is univocally determined by a characteristic of the proportioning valve, depending upon an input state from the brake lever at the front wheel. The relatively great decrease in a vertical load onto a road surface occurs at the rear wheel because of the specifications of a vehicle and the effect of a coefficient of friction of a road surface and others, compared with that at the front wheel which is not desirable in terms of a brake feel.
Normally, in a braking device for a motorcycle, a master cylinder is provided to a braking member such as a lever and a pedal and wheel braking means such as a brake caliper provided to a wheel are connected via a piping and the wheel braking means is operated by input-output characteristics univocally determined by the size and others of the master cylinder and the wheel braking means. See, for example, JP-A No. 264278/2000.
However, in this type of braking device for a motorcycle, when a large braking force is input to the side of a front wheel, a vertical load of a rear wheel decreases more because of the bite of the front wheel upon a road surface and the inertia force of a vehicle. When the vertical load of the rear wheel varies in braking, a braking feeling may be deteriorated.